Dazzle, by high intensity light sources, is a common problem in optical systems or imaging devices, causing damage to the sensor, degradation of image quality or loss of situational awareness for the user. The problem occurs when high intensity light sources such as sunlight, welding arc, car head lamps or lasers are directed at the system or device. Saturation or dazzle especially by laser is now a common problem, both in military and civilian environments, as lasers themselves have become smaller, cheaper and more readily available. This, in turn, has led to the need to provide such systems and devices with electro-optic protection measures (FORM) to limit or filter the transmission of light to the sensor. One approach to EOPM is to limit the effects of dazzle by spatial blocking using an optically addressed light valve (OALV).
One type of OALV works by coupling a photoresistor (PR) to a voltage dependent polarisation modulator, which may be a 90° twisted nematic, positioned between two polarizers. The polarizers may be set at predetermined angles relative to each other in order to remove a specific incoming wavelength. Both the PR and liquid crystal layer are situated between glass plates coated with transparent Indium Tin Oxide (ITO) to act as electrodes. A sinusoidal or square-wave voltage is applied to these ITO layers via wires attached using solid crystalline indium. The PR is a material whose bulk electrical conductivity can be altered by the presence of light. Typically photo-generated charges cause the resistivity to drop in response to light. In this way the voltage drop across the liquid crystal layer is determined by the intensity profile of light falling on the device.
One type of OALV incorporates a photoresistor fabricated from Bismuth Silicon Oxide (BSO). BSO has excellent photoconductive properties, including a high dark conductivity (conductivity in the absence of light), making it a perfect candidate for OALV's. However, BSO is itself optically active. This means that it causes a wavelength-dependent rotation of linearly polarized light passing through the crystal medium. This optical activity means that known BSO OALV's are effective at selectively limiting a single predetermined detrimental wavelength. Since different wavelengths are rotated by the PR by different amounts, the analysing polarizer needs to be set at a predetermined angle to block a specific incoming wavelength.
By incorporating a known OALV into an optical system or imaging device, such as a camera, night vision device, sight, safety goggles etc the sensor can he effectively protected from dazzle by one predetermined detrimental wavelength.